(1) Field of the Invention
This invention is in the field of water treating systems in which water typically as furnished by a water utility, or from wells, is softened and filtered by being passed through a regenerative filter including cation ion exchange media. More particularly, this invention relates to a multilayer filter for producing water which has been softened and from which substantially all visible particles have been removed.
(2) Description of the Prior Art
Hardness in water is the result of the presence of certain cations, notably those of calcium, CA.sup.2+ and magnesium, Mg.sup.2+. Hard water has many undesirable characteristics one of which is to form a precipitate with soap. When heated, hard water forms precipitates, the composition of which depends upon the anions present. A most common anion, particularly in surface water, is the HCO.sub.3.sup.- ion formed by the reaction of carbon dioxide CO.sub.2 with water H.sub.2 O. When a solution contains CA.sup.2+ and HCO.sub.3.sup.+ ions is heated, calcium corbonate CaCO.sub.3 forms. If the sulfate ion SO.sub.4.sup.-2 is present, crystals of calcium sulfate are formed which in certain circumstances produce a tightly adherent scale.
One of the oldest methods of softening water includes the addition of slaked lime Ca(OH).sub.2 and soda ash Na.sub.2 CO.sub.3. This method has lost favor because of its many economic disadvantages compared with the ion exchange method of water softening in which the water to be treated need not be analyzed, reagents need not be added in specific amounts, and there is no precipitate to be removed. The ion exchange method is applicable to large or small scale operations for the treatment of the water supply for the home or for industry. The initial ion exchange media were a class of substances known as zeolites which contain atoms of aluminum, silicon, and oxygen which are bonded into vast anionic networks, or lattices. The negative charges of the networks are compensated by a large number of Na.sup.+ ions which are positioned within the anionic lattice of a zeolite particle. Zeolites have been superseded as the ion exchange media in most modern water softening systems by organic cation exchange resins such as the crosslinked, polystyrene sulfonate cation exchange resins because such resins have a much higher cation exchange capacity as compared to the zeolites.
A significant advantage of cationic exchange media either an organic or a zeolite, is that such a media can be regenerated as it loses its effectiveness by having more and more of the Na.sup.+ ions in the lattices replaced by Ca.sup.2+ or Mg.sup.2+ from water as it is being softened. Regeneration is accomplished by passing a concentrated solution of sodium chloride through the cationic exchange media.
Water softening systems have heretofore concentrated on softening water by removing the Ca.sup.+2 and Mg.sup.+2 ions from water and replacing them with Na.sup.+ ions typically from an organic cation exchange resin. Other problems with the water supplied to the water softening system other than hardness include gases entrapped in the water, which affect odor, taste, and color, examples of which are chlorine and chlorophenols, as well as visible particles entrained in the water, including particles of iron and manganese. In addition the presence of iron or manganese in solution as well as the acidity of the water are problems which water softening systems have heretofore ignored. Where undersirable characteristics other than the hardness of the water were to be changed, they were changed by other apparatus or systems than those responsible for softening.
One problem associated with the use of an organic resin cationic ion exchange medium, such as crosslinked, polystyrene sulfonate cation exchange resins is that the particles, or beads, of the resins are substantially spherical. A bed, or layer, of such beads filters out particles of a size greater than 80 microns. Since particles of a size equal to, or greater than 30 microns are visible to the human eye, passing water through a bed of an organic cation exchange media does not remove substantially all visible particles or turbity. Such water is not polished, where polishing is defined as the removal of substantially all visible particles so that the water sparkles.